Zinc-based coated steel materials such as hot dip galvanized steel sheet, galvannealed steel sheet, and electrogalvanized steel sheet are widely used in automotive parts and particularly automotive parts constituting automobile bodies since these materials have just sufficient corrosion resistance in the environment of use of the automobile parts and are advantageous from the standpoint of cost. Among these materials, galvannealed steel sheet is manufactured by continuously subjecting steel sheet to hot dip galvanizing and then to heat treatment at a temperature of around 500-550° C. to cause mutual diffusion between the zinc layer and the steel substrate (base metal) so as to convert the entire coating layer into an Fe—Zn intermetallic compound layer. Compared to hot dip galvanized steel sheet or electrogalvanized steel sheet, galvannealed steel sheet has a coating layer which is electrochemically somewhat nobler, and its sacrificial anticorrosive ability is somewhat lower. However, the coating layer of a galvannealed steel sheet has improved adhesion to a paint coating which is formed thereon. For this reason, galvannealed steel sheet is widely used for automotive parts which are normally painted by electro-deposition coating following chemical conversion treatment. The coating layer of a galvannealed steel sheet is formed from Fe—Zn intermetallic compounds which are generally hard and brittle. Therefore, when such a sheet is subjected to press working accompanied by bending or drawing, a portion of the coating layer may cause powdering. In such cases, hot dip galvanized steel sheet or electrogalvanized steel sheet is used instead.
In recent years, there has been an increasing demand that automobile bodies guarantee safety during collisions. In order to cope with this demand, efforts to increase the energy absorbing properties of automotive parts at the time of a collision are being progressed. For example, efforts are being made to increase the energy absorbing ability at the time of a side impact by reinforcing a door with a side impact beam formed from a metal pipe such as a steel pipe by imparting a suitable curved shape over generally the entire length of the pipe, or by optimizing the shape or curvature of a reinforcing member which is installed inside a center pillar. For these purposes, processing techniques are being developed for bending members of a metal pipe and particularly steel pipe or pre-formed members of steel sheets into a shape suitable for automobile parts.
There is a strong demand for automobile parts to be lightweight and high strength in order to decrease the weight of automobile bodies so as to prevent global warming. In response to this demand, a high tensile strength steel having a strength level totally different from in the past such as a tensile strength of at least 780 MPa or 900 MPa or above is now being used. It is difficult to perform bending or similar working in a cold state on members formed from high tensile strength steel. Even when bending or similar working is carried out in a hot state, variations in shape due to nonuniform strains unavoidably develop, and there is a problem with respect to shape retention. In addition, in order to perform bending to an optimal shape, there is a demand for the development of bending techniques which can bend a steel material with high precision so as to form a bent shape which widely varies such as a shape in which the bending direction varies 2-dimensionally or 3-dimensionally.
In PCT/JP2006/303220, the present inventors proposed a hot bending method and apparatus which, as described below, can simultaneously and efficiently carry out bending and quenching of a material being worked using a roller die which can move multi-dimensionally even when carrying out continuous bending in which the bending direction of a steel material varies 3-dimensionally.
In this bending method, the steel material being worked is sequentially heated by a high-frequency induction heating coil to a temperature at which plastic working of the material being worked can easily be performed, or optionally to at least a temperature at which quenching of the material being worked is possible and at which the metal structure does not coarsen. The locally heated region is plastically deformed using a movable roller die and then immediately rapidly cooled. When carrying out this bending method, it is practical from the standpoint of manufacturing costs to use equipment which heats the material being worked in air.
As stated above, a steel material used in an automotive part is generally subjected to chemical conversion treatment and electro-deposition coating, and zinc-based coated steel materials are widely used in this application in order to increase corrosion resistance. Therefore, if zinc-based coated steel materials can be used in the bending method proposed in the above PCT application, a bent member or a hardened member having corrosion resistance can be manufactured while preventing oxidation of the steel base metal, and application of such coated steel materials to automotive parts can be strongly promoted.
However, heating of a zinc-based coated steel material to a high temperature at which quenching is possible (such as the A3 transformation point or higher) causes the following problems: (a) there is the possibility of zinc vaporizing during the heating process due to the fact that the vapor pressure of zinc, which is, for example, 200 mm Hg at 788° C. and 400 mm Hg at 844° C., rapidly increases as the temperature increases, (b) oxidation of zinc may occur during heating in air, and (c) there is the possibility of the coating layer disappearing due to the phenomenon that Zn dissolves in the ferrite phase of the base metal to form a solid solution, this phenomenon becoming significant when a zinc-based coated steel is heated to at least 600° C. and particularly to above 660° C. at which the Γ phase (Fe3Zn10) decomposes. These problems may cause the coating layer to be unable to perform its function.
Patent Document 1 identified below discloses a method of manufacturing a strengthened steel material by subjecting a steel sheet for induction hardening which has been galvanized to induction hardening which is carried out by heating and subsequent cooling such that the heating temperature is at least the Ar3 point and at most 1000° C. and that the heat cycle time from the start of heating until cooling to 350° C. is restricted to at most 60 seconds. According to this method, a hot dip galvanized steel sheet in which the base sheet is a steel sheet for quench hardening can be used to manufacture a strengthened member by induction hardening such that regions to be strengthened are hardened by induction hardening while the coating on the hardened regions remains. By limiting the Fe content of the coating layer to at most 35% (in this description, unless otherwise specified, percent means mass percent), an automotive part having excellent paint coatability and corrosion resistance can be provided.
Patent Document 1: JP 2000-248338 A